An EEG instrument is a clinical, multichannel device used to measure and display the brain waves of a subject. The subject's brain waves are monitored by the use of a number of electrodes placed in contact with the scalp in a predetermined pattern or montage (typically, of nineteen locations) which is well known in the art. The electrical activity generated by the subject's brain, i.e., brain waves, are detected as electrical signals by these electrodes and transmitted to a signal monitoring unit where they are amplified and processed into graphic representation for viewing and diagnostic interpretation.
For many years the state of the art of electrode design and placement was rather primitive in relation to advances in the processing and analysis of EEG signals. Location and placement of the electrodes on the subject's scalp was done by trial and error techniques which were cumbersome, time-consuming, and unreliable. The state of the art was advanced by the development of a headset for EEG tests having the electrodes mounted in their proper positions and actuatable in good electrical contact with the scalp, for example, as disclosed in U.S. Pat. Nos. 4,632,122 and 4,683,892 issued to N. Johansson, E. Eralp, and T. Itil, and U.S. Pat. No. 4,800,888 issued to T. Itil, D. Shapiro, E. Eralp, and N. Johansson, which are incorporated herein by reference.
As illustrated in FIGS. 1 and 2 herein, the EEG headset 10 of Johansson, Eralp, and Itil includes a helmet liner 12 with a plurality of electrodes 20 placed at predetermined positions about the liner 12. In order to accommodate each of the electrodes 20, apertures 22 are provided in the liner 12 and a resilient mesh 24 is used to hold the respective electrodes in position within the apertures 22. The mesh 24 permits ventilation of air through the liner 12, thus making the headset 10 more comfortable to wear, and its resilience allows the technician to oscillate the electrode tips to displace the subject's hair for placement in contact with the scalp. The electrodes 20 have respective tips 50 with ends that are to be placed in electrical contact with the scalp. Individual air-actuated cylinder and piston arrangements displace the respective electrode tips 50 against the scalp when air pressure is supplied through air conduits 70 fed by feed line 110 from an air pump 100. Conduits 53 are used to supply an electrolyte fluid or gel to the electrode tips 50 to ensure good electrical contact with the scalp. The electrolyte is supplied to the conduits 53 from a feed line 120 connected to an external electrolyte supply.
The EEG headset and air-actuated, electrolyte-supplied electrode assembly of Johansson, Eralp, and Itil provide a significant advance in the art of EEG instruments. However, it has been observed that the electrodes of the EEG headset, and of EEG instruments generally, are susceptible to sporadic signal spikes, high voltage artifacts, rhythmical slowing in the Theta wave range, and polarization. In addition, the headset and electrode assemblies require certain improvements to maintain them more stably yet flexibly on the subject's head.